The invention relates to an image reading apparatus capable of ensuring high tone quality.
There are many apparatuses of recent development, which photoelectrically read color documents, subject the read documents to various color image processing, and produce color image prints.
As shown in FIG. 10, such a color image reading apparatus produces color prints as follows. A color document is read by an image sensor 226 consisting of a multiplicity of photodiode arrays, the read data is converted to an 8-bit digital signal by an A/D converter 231, and the digital signal is subjected to dark correction by a dark correction unit 235 to eliminate sensitivity variations among respective pixels. The dark correction is such that a reference signal read in darkness is subtracted from the digital signal. The dark-corrected digital signal is converted to a density-correspondent digital signal while subjecting the signal corresponding to reflectance data to a logarithmic conversion process by a density converter 236. The density-converted digital signal is then subjected to shading correction by a shading correction unit 237, where a reference signal produced at the time of reading a white region is subtracted from the density-corrected digital signal. The shading-corrected signal is gray-balanced by an END conversion module 301, and the gray-balanced color signal is thereafter outputted to other conversion modules of an image processing system (IPS).
While a conventional A/D converter converts a detected signal into an 8-bit signal at an equal pitch, the density converter 236 converts 8-bit reflectance data to 8-bit density data to produce 256 tone levels based on a look-up table having such a logarithmic curve as shown in FIG. 11. As the signal is subjected to a curve changed conversion process or a bent conversion process by referring to the look-up table, a plurality of tone levels may, in some cases, be reduced into a single tone level in some low-density pixel regions whose conversion curve is dull, while a plurality of tone levels may be reproduced as desultory stepping levels in some high-density pixel regions whose conversion curve is sharp. As a result, even though detected as 8-bit data, such signal cannot produce 256 tone levels, impairing the reproduced tone quality. Once the reproduction of 256 tone data has been impaired at this stage, such impairment is imparted to subsequent image processing stages, eventually deteriorating the quality of produced images due to absence of 256-tone data. Thus, such shortcoming must in some way be prevented.
The invention has been made in view of the above circumstances.